The present invention relates to rock drilling and to drill bits therefor.
Larsson U.S. Pat. No. 4,607,712 discloses a rock drill bit of the impact type for top hammer drilling comprising a cylindrical body and a front surface provided with fixed peripherally spaced buttons and a central button. Normally such a conventional drill bit drills about 30 m before regrinding is necessary.
To improve such drilling performance it has been proposed to use diamond enhance bits for top hammer drilling. The results, however, have not been competitive compared to conventional cemented carbide insert bits. Diamond enhanced button bits might be most useful in hard to extra hard abrasive rock where diamond enhanced button bits have the advantage that they do not need to be reground, whereas conventional cemented carbide buttons wear more rapidly and thus need to be reground more frequently. Other advantages with diamond-enhanced bits are: even hole dimension, even penetration rate, less down time due to less frequent changing of bits, etc. Previous diamond-enhanced bits for top hammer drilling have often had about the same life-span as conventional cemented carbide bits.
However, bad utilization of the bits due to asymmetrical location of the diamond-enhanced buttons results in heavy wear of one or two gauge buttons that are more subjected to more load than the others. The life of such heavily wearing gauge buttons constitutes the effective life of the bit. One way to make the diamond-enhanced bits more competitive is therefore to place the gauge buttons symmetrically at the same distance around the bit periphery. The potential when placing the gauge buttons symmetrically at the same distance between each other is (from the wear point of view) judged to be about double the life span of a conventional cemented carbide-button or cemented carbide insert-bit (when drilling in hard to hard abrasive rock). A diamond-enhanced button comprises a cemented carbide button substrate with a PCD-layer (poly crystalline diamond) on the tip of the substrate.